Ring oscillators, as illustrated in FIG. 6, are commonly found on a wide variety of serializer/deserializer integrated circuits, and are very useful because they have a wide tuning range. However, as the data rates continue to increase, and serializer/deserializer circuits must operate with data rates from 1 Gb/s to 6 Gb/s or higher, even the tuning range of the traditional ring oscillator is insufficient. This is because the oscillator frequency of the ring is determined by the delay of the ring, which in turn is affected by the input capacitance and resistance.
Current approaches include adjusting the power supply to the ring, adjusting the current bias of the ring, and switching in additional stages for additional delay. Approaches that adjust the power supply or current bias undesirably affect other properties of the ring oscillator, such as output amplitude, rise and fall time, waveform symmetry, and device noise so as to degrade performance. The addition of stages for delay requires additional area in the layout. Because adding stages act to reduce the oscillation frequency, it does not act to increase the oscillation frequency.
Therefore, it would be desirable to provide a method and circuit for tuning a ring oscillator at high frequencies.